1. Field of the Invention
The present invention relates to a temperature sensor used for temperature detection and a method of manufacture thereof or, in particular, to a temperature sensor suitably used as an exhaust gas temperature sensor mounted on a catalyst converter or the like of an exhaust system of automobiles for detecting an abnormal temperature or detecting the deterioration of a catalyst.
2. Description of the Related Art
In the prior art, the temperature detecting apparatus described in Japanese Unexamined Patent Publication (Kokai) No. 9-126910 has been proposed as this type of temperature sensor. This conventional temperature sensor comprises a solid-cylindrical thermistor including a pair of electrodes (platinum) each in the shape of a cylindrical pipe. A pair of signal lines (wires for retrieving a thermistor signal) from a rear two-core pipe (sheath pin) are inserted into the hollow cylindrical electrodes. The electrodes and the signal lines are welded to each other. Further, a metal cap covering the thermistor is coupled to the outer cylinder of the two-core pipe.
This temperature detecting apparatus in which a pair of electrodes led out in one direction of a solid-cylindrical thermistor are connected to a pair of signal lines, respectively, arranged in the direction of extension of the electrodes is generally called a radial-type thermistor.
With the recent trend toward a higher responsiveness of sensors, the current demand is for a smaller diameter of a temperature detecting portion. For the temperature detecting portion to be reduced in diameter, a reduced diameter of the metal cap and hence the thermistor is naturally required. In the prior art described above, however, a platinum pipe embedded in the thermistor is used as a thermistor electrode, and the signal lines are inserted and joined in the pipe. Therefore, the pipe is larger in diameter than the signal lines by a size equivalent to the pipe thickness and the insertion gap.
On the other hand, the thermistor itself requires a volume sufficient for acquiring the desired resistance characteristic. Therefore, the portion embedded with the platinum pipe forms a dead space of the thermistor. An increased pipe diameter thus unavoidably increases the thermistor diameter. Also, as described above, once the diameter of the signal lines is determined, the pipe diameter and thickness are also determined. The reduction in thermistor diameter, therefore, has a limit.
For the thermistor to be reduced in diameter, the thermistor electrodes are required to be changed in shape from a pipe into a rod. A study conducted by the inventors shows, however, that a reduced diameter of the temperature detecting portion of a radial-type thermistor poses a problem depending on relative positions of a pair of rod-like electrode wires and a pair of signal lines. This problem will be described with reference to FIGS. 5A, 5B.
FIGS. 5A, 5B show a working model of a radial-type thermistor fabricated by the inventor. A pair of rod-like electrode wires 2 led out in the same direction from a solid-cylindrical thermistor 1 are connected to a pair of signal lines, respectively, arranged in the same direction as the electrode wires (welded at points A in FIG. 5A). Numeral 4 designates an outer cylinder of the two-core pipe. FIG. 5B is a sectional view taken in line C--C in FIG. 5A.
As shown in FIG. 5B, each of the wires 2 and a corresponding one of the lines 3 are coupled to each other on the same side. The electrode wire pair 2 is normally arranged on lines on both sides of a cylinder axis 1a in order to minimize the body size while securing the required thickness of the thermistor 1. The cylinder axis 1a of the thermistor 1, therefore, is offset from a line connecting the signal lines 3 (by the distance indicated by numeral 5 in FIG. 5) and deviates from the center axis 4a of the outer cylinder of the two-core pipe. A cylindrical metal cap and an insulating member are arranged around the thermistor 1. The eccentric structure of the thermistor 1 described above requires a large diameter of the metal cap and the insulating member, thereby substantially increasing the size of the temperature detecting portion.
Also, the axes 1a, 4a would coincide with each other if the electrode wires 2 and the signal lines 3 are welded at the ends thereof. welding ends to ends, however, makes it difficult to secure the reliability of the junction. Especially, this structure is not desirable for the exhaust gas temperature sensor or the like of automobiles exposed to vibrations and other external forces.